Recently, a great deal of attention has been directed to the harmful effects of ultraviolet (UV) radiation on humans. Much of the attention has concerned the effect of such radiation on the eye. Accordingly, the value of strong UV absorption by eye glasses has been recognized.
It is well known that UV radiation can also cause degradation and discoloration in such items as paints, fabrics and plastics. Therefore, strong UV absorption by architectural glazing materials is beneficial. The sun is not the only light source that emits UV. Various types of artificial lighting, such as halogen lamps, may also emit UV radiation. Accordingly, there is an interest in minimizing UV radiation emitted by artificial sources as well. This may be achieved by utilizing UV absorbing glass in the fabrication of lamp envelopes, reflectors and lenses.
It is common knowledge that photochromic glasses are activated by absorption of UV radiation. The most evident utility of such glasses has been in control of visible light transmission. Inherently, however, they also strongly influence the intensity of UV transmission. This behavior is readily understood in terms of the Grotthus-Draper Law which states that: Only light that is absorbed can produce chemical change.
Photochromic glasses containing silver halide crystals absorb strongly at wavelengths shorter than 320 nm, but only absorb weakly in the interval between 320 and 400 nm. Even though radiation in the wavelength range of 320-400 nm is much less eliminate transmission of this radiation as well. Therefore, several suggestions have been advanced for accomplishing this. For example, it has been proposed to dope the above glasses with ions which provide additional absorption of UV radiation.
Photochromic glasses containing halides of copper and/or cadmium are also known, but not commercially available. Such glasses were originally disclosed in U.S. Pat. No. 3,325,299 (Araujo I). The transmission cutoff in these glasses occurs at approximately 400 nm, and is much sharper than that in silver halide glasses. Consequently, protection against UV radiation is complete in these glasses without additional doping. The precipitation of the copper halide phase in these glasses is like that of the silver halide phase in the silver halide photochromic glasses. It may require heating of a glass containing in solution the copper and halogen ions of interest. As taught in the patent, the glass is maintained for a short time at a temperature somewhat above the annealing point.
U.S. Pat. No. 4,166,745 (Araujo II) discloses copper-cadmium photochromic glasses that have a refractive index of 1.52-1.54, and that may be strengthened by an exchange of sodium ions for lithium ions.
U.S. Pat. No. 4,222,781 (Morse et al.) discloses photochromic glasses based on copper halide wherein good optical clarity and photochromic properties are provided by controlling the alkali metal oxide, the Al.sub.2 O.sub.3 and the B.sub.2 O.sub.3 concentrations in the base glass, and/or by adding MoO.sub.3 or WO.sub.3 to the composition.
European Publication Number 0 456 351 A2 [U.S. Pat. No, 5,145,805] (Tarumi et al) discloses two glass families containing up to 15% copper halide. The non-phosphate family comprises, in percent by weight, 20-85% SiO.sub.2, 2-75% B.sub.2 O.sub.3, up to 15% Al.sub.2 O.sub.3, up to 30% alkali metal oxides, up to 10% divalent metal oxides and up to 10% of at least one of ZrO.sub.2, La.sub.2 O.sub.3, Y.sub.2 O.sub.3, Ta.sub.2 O.sub.3 and Gd.sub.2 O.sub.3.
There are numerous applications for glasses having the sharp UV cutoff inherent in the copper or copper-cadmium halide glasses. Frequently, however, such applications require avoiding any change in visible absorption such as occurs in photochromic glasses exposed to UV radiation, e.g., sunlight. Many UV materials exhibit yellow color which is unacceptable for certain applications. U.S. Pat. No. 5,322,819 (Araujo III) disclosed a non-photochromic, copper halide containing UV absorbing glass which exhibits a sharp cutoff in ransmission in the wavelength interval between visible and UV radiation. Specifically, the Araujo III reference ('819) disclosed a non-photochromic R.sub.2 O--B.sub.2 O.sub.3 --SiO.sub.2 glass which contains a precipitated cuprous or cuprous-cadmium halide crystal phase and has a sharp spectral cutoff at about 400 nm, the glass composition consisting essentially of, in cation percent, 35-73% SiO.sub.2, 15-45% B.sub.2 O.sub.3, 0-12% Al.sub.2 O.sub.3, the Al.sub.2 O.sub.3 being less than 10% when the SiO.sub.2 is over 55%, 0-12% Li.sub.2 O, 0-20% Na.sub.2 O, 0-12% K.sub.2 O, the Li.sub.2 O+Na.sup.2 O+K.sub.2 O being 4.75-20%, 0-5% CaO+BaO+SrO, 0.125-1.0% Cu.sub.2 O, 0-1% CdO, 0-5% ZrO.sub.2, 0-0.75% SnO.sub.2, 0-1% As.sub.2 O.sub.3, and/or Sb.sub.2 O.sub.3, the glass containing 0-1.25% Cl, 0-1.0% Br, 0.25-2.0Cl+Br and 0-2% F by weight.
UV absorbing glasses are generally available in bulk form, making then impractical for certain applications such as UV absorbing paints and varnishes as well as UV absorbing skin creams for example. Accordingly, it is a principal object of the present invention to provide UV absorbing glass in a form which can be readily utilized in such applications.